Humidity and Inspired Oxygen Concentration During High-Flow Nasal Cannula Therapy in Neonatal and Infant Lung Models

High-flow nasal cannula therapy (HFNC) for neonate/infants can deliver up to 10 L/min of heated and humidified gas, and F_IO can be adjusted to between 0.21 and 1.0. With adults, humidification and actual F_IO are known to vary according to inspiratory and HFNC gas flow, tidal volume (V_T), and ambient temperature. There have been few studies focused on humidification and F_IO in HFNC settings for neonates/infants, so we performed a bench study to investigate the influence of gas flow, ambient temperature, and respiratory parameters on humidification and actual F_IO in a neonate/infant simulation.

HFNC gas flow was set at 3, 5, and 7 L/min, and F_IO was set at 0.3, 0.5, and 0.7. Spontaneous breathing was simulated using a 2-bellows-in-a-box model of a neonate lung. Tests were conducted with V_T settings of 20, 30, and 40 mL and breathing frequencies of 20 and 30 breaths/min. Inspiratory time was 0.8 s with decelerating flow waveform. The HFNC tube was placed in an incubator, which was either set at 37°C or turned off. Absolute humidity (AH) and actual F_IO were measured for 1 min using a hygrometer and an oxygen analyzer, and data for the final 3 breaths were extracted.

At all settings, when the incubator was turned on, AH was greater than when it was turned off (P < .001). When the incubator was turned off, as gas flow increased, AH increased (P < .001); however, V_T did not affect AH (P = .16). As gas flow increased, actual F_IO more closely corresponded to set F_IO . When gas flow was 3 L/min, measured F_IO decreased proportionally more at each F_IO setting increment (P < .001).

AH was affected by ambient temperature and HFNC gas flow. Actual F_IO depended on V_T when gas flow was 3 L/min.